No license is needed to write software that will interface with the TMCC or Legacy command base using the protocols that Lionel has published, and Dave will provide you with the LCS protocol for the asking. It's very easy to write Arduino code that will control the functions of a TMCC or Legacy engine (or DCS, for that matter, with Mark Divecchio's work).
So if you're writing your own system from scratch - which it sounds like you will, if you want to implement training data and rules - you might consider a platform-agnostic approach for the hardware interface.
Legacy does not provide 2-way communication like Railcom might, although you could investigate the capabilities of the LCS sensor track. But if you need to rely heavily on Railcom then only DCC will be an option.
